1. Field of the Invention
The present invention relates to a receiver device for a mobile radiocommunication unit employing a speed estimator.
The invention relates more particularly to the field of telecommunications and especially to the field of radiocommunication terminals.
2. Description of the Prior Art
In radiocommunication terminals, the signals received by the receiver of a mobile receiver unit are degraded because of propagation channel variations. The propagation channel variations depend mainly on the speed of the mobile receiver unit. The channel variations lead to a channel estimation error. The unwelcome consequence of this is that the bit error rate is significantly degraded when the received signal is decoded. Also, a propagation channel estimator can be provided in the structure of the receiver of the radiocommunication terminal in order to take account of amplitude variations of the signal received by the antenna of the receiver due to the speed of the mobile receiver unit.
However, the propagation channel estimator is insufficient for determining the impulse response of the channel with good accuracy. When the speed of the mobile receiver unit increases, the propagation channel varies too quickly for the propagation channel estimator to be able to estimate the frequency and phase variations with sufficient accuracy.
An alternative set out in the patent document GB 2 276 064 consists of using Wiener filtering in the receiver. A Wiener filter is a digital filter with a finite impulse response. The amplitude of the output signal of a Wiener filter is closely related to that of the input signal. In other words, a Wiener filter is a filter in which the output signal at a given time depends only on the input signal at that time.
To alleviate the problem of propagation channel variation and the resulting degraded receiver signals, the patent previously cited discloses the use of a plurality of Wiener filters, each set for a range of contiguous speeds of the mobile receiver device. Selector means select as a function of the speed of the mobile receiver unit the appropriate Wiener filter for maintaining the best possible radio link. Selection is based on detecting the Wiener filter that has the highest output power. According to the teaching of the patent previously cited, it is necessary to use all the Wiener filters of the receiver in parallel and then to select only the filter with the highest output power. Thus all the Wiener filters of the receiver need to be operating at the same time in order to select the right filter.
This makes the solution proposed by the patent document previously cited very complex.
Also, the object of the invention is to alleviate the drawbacks of the prior art by proposing a receiver device whose complexity is very greatly reduced, whilst improving the propagation channel estimates regardless of the speed of the mobile receiver unit.
In fact, for any radiocommunication system, knowing the speed of the mobile receiver unit is very important for improving the quality of service. The speed of the mobile receiver unit induces propagation channel variations which have a direct impact on the channel estimate and consequently on the bit error rate.
To achieve the above object, the invention proposes a receiver device for a mobile radiocommunication unit that includes, on the one hand, a bank of Wiener filters each set for a particular range of speeds and, on the other hand, a mobile receiver unit speed estimator so that the appropriate Wiener filter can be configured automatically and dynamically as a function of the speed of the mobile receiver unit. The speed estimator provides a speed indication to the bank of Wiener filters and therefore enables the Wiener filter suitable for the speed of the mobile receiver unit to be selected. Thus only one Wiener filter operates at a time.